1. Field of the Invention
The present invention relates to an OFDM signal transmission system used for digital terrestrial broadcasting and electronic commerce using communications.
2. Description of the Related Art
Regardless of the type of a transmission path such as satellite, cable and terrestrial, broadcasting is being digitalized more and more on a worldwide scale in recent years. Of these digital broadcasting systems, the European and Japanese digital broadcasting systems use an Orthogonal Frequency Division Multiplex (hereinafter referred to as “OFDM”) transmission system.
The OFDM transmission system modulates multiple carriers orthogonal to one another with information to be transmitted for every symbol period, multiplexes those modulated signals and transmits the multiplexed signal. When the number of carriers used increases, the symbol period of each modulated signal becomes extremely long, and therefore the OFDM transmission system is characterized in that it is less susceptible to multi-path interference.
Furthermore, the OFDM transmission system provides a redundant period called a “guard period” for every symbol period, thereby prevents interference between symbols and further enhances resistance to multi-path interference. Such a redundant period can be provided because the symbol period of an OFDM signal is extremely long and a reduction of the transmission capacity due to the addition of the redundant period can be confined within an allowable range.
FIG. 1 is a schematic view showing a configuration of an OFDM signal. Hereinafter, a period necessary to transmit information will be referred to as an “effective symbol period and the effective symbol period plus a guard period” as a whole will be referred to as a “symbol period”. As shown in the hatching in the figure, the guard period of the OFDM signal is a cyclic copy of the last part (copy part) of the effective symbol period.
Then, FIG. 2 will explain how the guard period increases the resistance to multi-path interference. A desired signal or a delay signal in the figure denotes an OFDM signal that has arrived with a time difference of τ and G1 and G2 denote guard periods of the first and second symbols respectively and S0, S1 and S2 denote effective symbol periods of the 0th, first and second symbol respectively. Here, the desired signal and the delay signal receive different symbols during period A and period B and receive the same signal during period C. That is, as far as time difference τ is shorter than the guard period, interference between symbols by the delay signal remains within the guard period and never adversely affects the effective symbol period of the desired signal.
Using the OFDM transmission system as the transmission system for digital terrestrial broadcasting makes it possible to implement an SFN (Single Frequency Network) that constructs a relay network using a single frequency by capitalizing on this feature of high resistance to multi-path interference and use frequency resources effectively.
FIG. 3(a) shows a case where a large-scale SFN is constructed using a high power relay station and FIG. 3(b) shows a case where a small-scale SFN is constructed using a small power relay station. When FIG. 3(a) is compared with FIG. 3(b), the distance between relay station 1A and relay station 2A is larger in FIG. 3(a) and the time difference until a broadcast signal arrives from the respective relay stations at reception point 3A is also greater. Therefore, constructing the large-scale SFN shown in FIG. 3(a) requires a longer guard period than constructing the small-scale SFN shown in FIG. 3(b).
However, taking a longer guard period requires an increased redundant time accordingly, and for all the increased symbol period based on the OFDM transmission system, reducing the transmission capacity poses a problem when high definition television (hereinafter referred to as “HDTV”) video signals with a high volume of information are broadcast.
Therefore, when minimizing the reduction of transmission capacity due to a guard period to construct a large-scale SFN, it is advantageous to take a longer symbol period.
On the other hand, the digital terrestrial broadcasting system in Japan adopts Differential Quaternary Phase Shift Keying (hereinafter referred to as “DQPSK”) or time interleave that scatters data of symbols adjacent in terms of time as the modulation system of each carrier, and thereby allows stable reception even in a mobile unit reception environment where the transmission path characteristic changes with time.
In this case, a shorter symbol period is less susceptible to time variations and allows stable reception performance even during a high-speed movement.
Thus, an optimal symbol period length varies depending on the service contents when HDTV video images are broadcast using a large-scale SFN or when services are broadcast to a mobile unit traveling at high speed, etc.
In order to respond to such a demand, as shown in FIG. 4, the digital terrestrial broadcasting system in Japan provides three types of mode with different effective symbol period lengths and four types of guard period ratio (ratio of guard period length to effective symbol period length) for the respective modes. Hereafter, this combination of a total of 12 types will be referred to as “transmission mode”. Of these types, for example, both the guard period ratio ⅛ and the guard period ratio ¼ in mode 3 have the same guard period length of 126 μsec.
Once the locations of relay stations of a broadcaster are determined, it is possible to estimate a maximum value of differences in time required for a broadcast signal to arrive from each relay station at a reception point within the service area and a necessary guard period length is determined from the numerical value. This numerical value varies from one relay network to another, and therefore the guard period length and the accompanying effective symbol period length may vary depending on the region or broadcaster etc.
Furthermore, when the guard period length determined by a relay station is, for example, aforementioned 126 μsec, the broadcaster can select whether the guard period ratio in mode 3 should be set to ⅛ or the guard period ratio in mode 2 should be set to ¼. At this time, it is also possible to switch between these two transmission modes depending on the service content of a program, for example, using the guard period ratio ⅛ in mode 3 to increase the transmission capacity for a program broadcasting HDTV images and the guard period ratio ¼ in mode 2 to provide services to high-speed mobile units stably for a program broadcasting services for mobile units.
Furthermore, after a broadcasting service is started, when the distance between relay stations decreases due to additions of relay stations and the aforementioned time difference decreases, the necessary guard period length also decreases. However, since the guard period is a redundant period which would originally be unnecessary for transmission of information, the guard period length is naturally changed to a minimum necessary length from the standpoint of effective use of frequency resources.
As shown above, the transmission mode expressed by a combination of the effective symbol period length and guard period ratio may vary depending on the region or broadcaster and a certain broadcaster may also change with time.
On the other hand, demodulation processing for an OFDM signal on the receiving side extracts only the period necessary for demodulation from a received signal, applies Fast Fourier Transform (hereinafter referred to as “FFT”) to the signal, thereby separates the carriers sent after being multiplexed and then applies detection processing according to each carrier modulation system. In the process of that processing, the transmission mode such as an effective symbol period length and guard period ratio constitutes indispensable information.
Thus, when the transmission mode of a received signal is unknown, the method for automatically acquiring the aforementioned transmission mode information from the received signal itself through signal processing is disclosed in Patent Gazette No. 2863747 or Patent Gazette No. 2879034, etc.
The prior arts disclosed in these literatures take advantage of the fact that a guard period of an OFDM signal is a cyclic copy of a signal at the tail of an effective symbol period, calculate a correlation between the received signal and a signal obtained by delaying the received signal by an estimated effective symbol period length, analyze the waveform of this correlation signal, and thereby decide the effective symbol period length and guard period length.
Conventionally, people have been conducting various kinds of electronic commerce, for example, purchasing products using a communication network such as the Internet. In such electronic commerce, products to be traded are posted on a homepage, for example. The user accesses this homepage from his/her own terminal over the Internet, selects a product to be purchased and at the same time enters user information that identifies the user.
Then, the user enters a credit number and an ID during a settlement period and specifies a settlement method such as payment on delivery. A homepage operating organization (center) authorizes the placement of an order for the product upon completion of such entries and a supplier that has received the order delivers the product to the user and receives payment according to the specified settlement method.
However, according to the transmission system using the aforementioned OFDM system, an OFDM signal is received and then the transmission mode is decided from the received signal, and therefore the time after the user selects desired information until the user receives the information includes a time for the above-described decision, which prevents quick response to the user's demand.
Furthermore, when a broadcaster switches between transmission modes according to the service content of a program as described above, demodulation processing is temporarily broken up immediately after the switchover and it is not until transmission mode decision processing is recovered from that state and the decision result is obtained that it is possible to output information after the switchover of the transmission mode, and therefore a supply of information to the user is suspended for a long time.
Furthermore, the aforementioned electronic commerce using a communication network involves problems concerning security and complexity of settlement. With regard to the security with the use of a credit card, there is concern about falsification, etc. With regard to settlement, settlement with a simpler procedure is accompanied by lower reliability of security. On the other hand, attempting to achieve both the security and ease of settlement results in an increase in the scale of system configuration.